UPSLOPE-MIGRATING-BEDFORM DEPOSITS IN SUBMARINE CHANNELS: INTEGRATING SEAFLOOR OBSERVATIONS AND THE STRATIGRAPHIC RECORD (Invited Presentation)

Recent technological advancements have greatly improved our ability to image the seafloor and monitor downslope sediment gravity flows, which has provided novel insight into enigmatic deep-water sedimentary processes. For example, Froude-supercritical bedforms (e.g., antidunes, cyclic steps) are now commonly identified in modern channelized subaqueous environments and are considered to be important geomorphologic and stratigraphic components of deep-water depositional systems. However, the presence and geomorphologic characteristics of these bedforms, what flow processes they are associated with, and how they are preserved in the stratigraphic record remain topics of ongoing investigation. This study examines repeat bathymetric datasets from two well-constrained deep-water channelized environments (Squamish Delta, British Columbia and Monterey Canyon, California) in order to investigate the geometries of supercritical bedforms, their spatiotemporal evolution, and ultimately, their stratigraphic products. These results from the modern seafloor are then applied to aid in interpretation of Late Cretaceous submarine channel strata of the Nanaimo Group on Gabriola Island, British Columbia.

Despite differences in environmental setting, similar bedform morphologies were observed in the Squamish and Monterey systems, with wavelengths between 10 – 100 m and heights between 0.5 - 3.75 m, respectively. Stratigraphic reconstructions using multiple repeat bathymetric surveys indicate significant reworking of bedform deposits between survey time intervals, resulting in flute-like and lenticular, erosion-surface-bound sedimentary bodies that are 7 - 60 m long, 10 – 80 m wide, and 0.3 – 2.5 m thick. The scale and geometry of deposits likely reflect the size of the flow event, bedform dimensions and migration history, and the overall aggradation rate. Architecturally and dimensionally comparable units are preserved within outcrops of the Nanaimo Group supporting the interpretation of a low aggradation setting dominated by supercritical flow transitions. These results help establish geometric criteria and relationships for supercritical bedform deposits that may support more refined depositional models and paleoenvironmental interpretations from deep time strata.